Hi everybody,

I have just a quick question for those 55ax owners out there. What is a reasonable rpm, at sea level, for a 55ax turning a 11x5 APC prop on 10 to 15% fuel?

Thanks,

ser00

First off, an 11x5 is a bit too small for a 55AX. 12x7 is the lowest recommended size - Now of course you can go a little smaller, but 11x5 is small enough to make the RPM very high (Like maybe 13,000 - 13,500? But that's just a guess). In any case, you should try to prop it to get 10,500 - 11,500. Too fast and you may need to replace it years before you had intended

I agree, 11x5 is way to small. If you want the low pitch try a 13x4 or maybe a 13x5. I've rand the .55's successfully with 11x8, 12x7 and 13x4. Never bothered to look at the RPM. Just set them a few RPM rich of peak by ear and have never had an issue.

I'm not at sea level, ~3450'. My 55 was much too busy sounding with the APC 11x5. 12x6 was too much load. Sweet prop is the APC 12x5 on my 4*40. Low idle, instant transition, fast, unending fast vertical. Doesn't use much fuel either, but I'm not at full throttle all the time. If memory serves, ~ 12,400 flying rpm.

i run a ax55 on a 11x5 and it runs rather well. you will not wear out your engine any faster turning it at 14500 rpm compared to 12000. if you are turning a ax55 less than 12000 rpm your prop is to big

I disagree. All you will get out of that prop setup on that engine is more noise. The wear and tear are a matter of fact. That's why OS recommends the starting prop that Minnflyer suggested.


CGr.

i have used an 11x6 apc....i also liked a 13x4 for 3d and a 12x6 with a tuned pipe

Yeah, good choice.. low pitch good diameter. And, it's a quiet combination.

CGr

SPECS: Displacement: .545 cu in (8.93cc)
Bore: .906" (23mm)
Stroke: .846" (21.5mm)
Practical RPM: 2,000-17,000
Power Output: 1.75 ps/16,000 RPM
Weight: 14.29oz (405g) w/o muffler
18.52oz (525g) w/ muffler
Recommended Props: 12x7-8, 13x6-7
I run my 55 with a APC 9X8 and a Jett muffler and it turns 16800 rpm in a CG Skylark 56. I've tried the recommended props (with the power box muffler) and it was a dog to me. Flying the 55 with a 11X5 would be a great combo if your into 3D or aerobatics. As far as premature wear due to high rpms, as long as you use a fuel with castor oil ( I add 3ozs to each gallon Powermaster or Omega) you will be OK: I've never wore out an airplane engine and I have 3 OS 77 ducted fans that are over 20 years old. I will admit to wearing out a couple boat motors but I was running 65-75% nitro in them.

Using a Xoar 13x4 for 3D. Getting 11000 ready to fly. A little less than 2 gallons in.

Hi everybody,

Thanks for all the responses. The reason I ask is I have two 55ax(s), one in an ultra sport 40, the other in a MKI P-51 PTS from Hangar 9. Both planes are swinging APC 11x5s. The performance of the Ultrasport and its 55ax is stellar. It is fast and has good vertical performance (but probably not unlimited, unless it comes to a crawl and goes vertical slowly inch by inch). The P-51's performance is not so stellar. It is not very fast and vertical is not really there. Now, I know that this is comparing apples to oranges. I usually tach my 55s around 11.5k rpms on the ground. I do not believe in *****g out the engines, thus shortening their lives. The p51 has about about 170 flights on it and it still has incredible compression. I did not tach the Ultrasport 55ax but I believe it is higher, maybe 12.5k. One thing that I notice is that on the p51, the wing, where the exhaust hits it, looks like a BP oil rig sits there. It is covering in oil. I am thinking that I should probably slowly lean out the engine until I get a more reasonable amount (read lesser) of oil. Therefore, I wonder what I should be taching the 55s at. I typically have a rule of thumb for props and engine sizes: 10" for .40s, 11" for .50s, 12" for .60s, 13" for .70s and 14" for .90s. Of course your mileage may vary on type of plane and type of flying. Also, OS seems to be a little enthusiastic when it comes to propping their AX line of engines. The 55ax will perform like a dog with a 12x6 or 13x5. I have tried them and seen another use a 12x6 on a Wild Hare with so-so performance. But I have used the 55ax and 12.25x3.75 APC in a Dazzler and it was a great performer with unlimited vertical (and we are talking about fast verticals). Kind of a pain in the butt to land with that heavy engine on the front. As far as in wear, I can only comment that my two 55ax(s) seem like new, even with 170 flights and 80 flights on them. But I do take very good care of the engines and do not max peak them for that last ounce of performance. I also add castor oil to my fuel (run about 20-22% oil synth/castor oil) and the bearing still look shiny and smooth (I have heard that OS bearings are total crap and do not last long, especially from one guy in our club, but his OS two strokes make my 2-strokes sound like 4-strokes if you know what I mean!)

Happy flying and soft landings!

SER00

Well, look at it this way. The oil residue is just that - Oil. The oil does not burn, that's why it's spit out. If you lean the engine, it'll turn higher RPM, but it'll still spit out the same amount of oil.

In other words, by leaning out the engine, you may be putting less oil through the engine per stroke, but you're getting a lot more strokes, so it will be about the same.

As for your prop theory, that is what you call a rule of thumb. It may work, but it's far from accurate. I run a graupner 12x7 on my 55AX and it's far from a dog with 15% Cool Power.

Wow, so many misconceptions in a single thread. Where to start???

First off, to generate power in a 2 cycle model engine, it requires RPM. OS recommends rather heavy props for their AX line, which I think is more marketing than reality. Not to say that you can't run a 13x7 prop, but other than being quiet there is nothing to recommend this approach. With a heavy load, the main needle valve becomes quite critical with a very narrow range between rich and lean. And since the rpm will be very very low (<10,000), you will not be making very much power, certainly far less than 1.75 PS it is rated at when at 16,000 rpm. At 10K or less I doubt the actual power output would be above 1.0 horsepower.

As to the amount of oil exhausted and the effects of RPM, most of the oil content is for the rod bushings, not the piston and sleeve. As you run an engine at lower rpm, the rod actually need more oil than an engine that runs at a much greater rpm.

While a 11x5 APC does sound a bit light for the engine, it will not harm the engine or shorten it's life in any measurable way, as long as it not run lean. But because the needle will be so much broader, it will be very easy to needle for a successful flight. The 12X5 APC prop would be my pick along with an 11x7, depending on airplane drag.

I would suggest those that normally run their AX 55 with higher loads to experiment a bit more with lighter props that get the rpm into the 12K+ range and report back their findings on the airplanes performance.

Yup, misconceptions all over the place.

First one is that an 11x5 actually is an 11x5. OK, it is and it isn't. The mfg certainly printed it on the hub and sells it as such but.........

The point is that there are probably 8 or 10 11x5s sold today, and the performance you get out of the lot is quite a broad range.

To ask what rpm you can get out of propellers with just the diaXpitch to go by is worse than going by rules of thumb.

We recently ran some tests using about 6 props, all the same dia/pitch on an electric. A couple loaded the motor such that it drew way too many amps and one wouldn't reach the design rpm for that motor. We shut that sucker down quick, 'cause that is a burnout about to happen. A couple didn't draw amps worth spit. All props the same, right.

You wanna' know what works to answer questions about props for model airplanes? A handful of props and a runway. OK, and the time to test each prop on the engine/airplane you want to fly with a decent prop on it. Everything else, rules of thumb and rules of toes especially, is just BS against BS. Theory against theory.

In theory, practice and theory are the same. In practice, they seldom are.

BTW, when you're filling your hands with the props to test, pick a couple that aren't recommended by the prop experts. You'll surprise yourself.

I run mine with a APC 11X6, no baffle, and just let it sing. It's nice and happy and the throttle response is good. Around 13 to 13.5k. My issue would be how rich I would have to run it to get only 11.5k with an 11X5 and how much damage I would be doing to the motor running it that rich, it's not a Fox 35!

I'm thinking the "oil rig" is more unburned fuel than anything else. My guess would be that you will burn up any ABC/ABL engine far faster running that rich than running at 14 to 15k every run. How many people have ruined their ABL engines just by running them too rich durring break in.

I agree. I usually ask the plane to tell me which is the "best" prop. Works every time.

Just to get you up to speed, no one is suggesting running the engine rich to get to a specific rpm, but rather changing the load of the prop to operate the engine at a different rpm range when set to correct mixture.

I use a 12.25 X 3.75 APC with my 55 AX on my super decathlon, haven't checked the RPM's but have unlimited vertical and it will even hover with this prop.

Well, there are some clubs that have noise restrictions. Running an engine at a higher RPM than necessary can cause more noise than is acceptable. We have a rather sensitive area around our field, although it is in the middle of the boonies, we have neighbors. All it takes is to irritate one and we're out.

So, it becomes a toss up. Noise and lots of power, and all that goes with it, or a bit quieter operation and a bit less power, but satisfied neighbors and we keep our club.

Not all clubs have these issues. We do, and I'm sure others do to.

I've said it before and perhaps you all have read that in my posts. This is the main reason that I oppose high RPM's and loud running aircraft.

CGr.

I have a 55AX in my SPAD Talon and I have it set up for speed. I have tried all sorts of different props on it and by far my favorite is the evolution 11x6. I have the Tower Hobbies pipe on it and it will pull this heavy plane straight up all day long but idles nice and slow. I have been a big fan of the evolution props for my sport planes. Plus the Tower muffler sounds awesome! Wouldn't reccomend it if you are in a noise sensitive are like CGRetired though. It is LOUD!

Hi everybody,

Wow, I never expected to get this much of a response. First, to Minnflyer, the prop size according to engine size is just that, a quick and nasty rule of thumb. As I said, individual mileage may vary. It really does depend on the plane and flying type. I would prop a Sig Hog Biplane differently than I would a Venus or a Patriot, even if they had the same engine size. I also see your point about the oil being the same no matter what (I had a dumb moment there). Oil is the same through out since it doesn't burn (as long as I am not running the engine too lean). If anything, I am probably wasting fuel more than anything else. Also, running the engine at 11.5k rpms is not going to damage the engine. It is running well into 2 stroke performance, not like a rich break-in "four stroking" performance which in my opinion can cause more damage. I also agree that the best determination for the correct prop is to test out several different sizes and pitches and choice the one you are most happy with. Now, I do not pretend to be a "Know it all." I throw my statements out there for more learned and experience people here to read and correct me when needed by sharing their experiences. Believe me, I have learned a lot in two years, and by reading these forums, I have the collective wisdom of many years of experience. I will probably slowly increase the rpms on the P51 until I get to a point where I am satisfied with the performance. I will do it slowly and responsibly. I will probably stop around 13.5k rpms (unless it seems that the engine is too lean) and switch to a different prop pitch or size (11x6 or 12x5).

Happy flying and soft landings!

ser00

As long as it doesn't make to much noise... [X(]

Had to get that in.

CGr.

I find as a general rule, the more noise they make the longer they last Take Harleys for example.......

Sorry CG, couldn't resist